A Giardia lamblia adult mouse model was used to understand mechanisms involved in antigenic variation. Both immune and nonimmune factors are important. In one clone expressing one variant specific surface protein (VSP), antigenic variation occurs in both immunocompetent and nude mice, but fails to occur in SCID mice. However, some clones expressing other VSPs are not continually expressed in SCID mice indicating that VSPs are preferentially maintained or lost by the host. Prior studies used encystation of Giardia lamblia as a means of understanding a relatively simple developmental process. A number of studies showed that protease(s) are essential for encystment to occur. A series of studies were initiated to understand control of nuclear division in Giardia and how structural proteins are altered during encystation and division. Six Mabs to different structural proteins were used to identify the corresponding genes, the homologue of the cyclin-dependent kinase 1 gene in Giardia identified, and antibodies made to it. A Zn finger active compound was found to be effective in the treatment of animals infected with Giardia lamblia.